Arrow
Jacqueline MacInnes Wood might have been the sinister Black Canary the pilot episode of Arrow, but she didn't get to revel in her new gig for long. She was too busy to continue with the show. She just had too many other acting roles lined up! Caity Lotz was hired to take over the position. Now that's a character who can successfully pull off an eye mask!
The Office
Only true office fans would ever notice this change. The character of Pam has had two different mothers appear on the show. The second mom is Linda Purl, remembered by fans for her uncomfortable escapade with Michael. In the first iteration of her, Shannon Cochran played the parent (and looked entirely different...)